1. Field of the Invention
The present invention relates to a sheet with anti-counterfeit functions where authentication information that is difficult to counterfeit for a third a party trying to counterfeit or rewrite with ill intentions or for a third party trying to sell a counterfeit is recorded, and a manufacturing method for the same, as well as an article, an authentication card, a bar code label and an authentication system.
2. Description of the Related Art
Authentication information is recorded in credit cards and ID cards, and whether these are authentic or fake is determined by a magnetic recording portion that is provided on the rear surface of the card, or a hologram that is attached to the front surface of the card. Authentication by means of a hologram image is, for example, disclosed in the US Patents that are cited as the following U.S. Pat. Nos. 5,574,790 and 5,393,099.
In addition, a latent image that cannot be seen with the naked eye without an intervening polarizing plate is formed on a layer that is formed of a polymer liquid crystal material, and a reflective layer is formed on the lower surface thereof in the passport that is disclosed in Japanese Patent Application Laid-open No. 2001-232978. This is irradiated with polarized light, so that reflected light can be observed via a polarizing plate, and thereby, a pattern that is formed as the latent image is authenticated in accordance with the disclosed method.
In addition, as for means for forming a latent image on a retardation film, as is disclosed in Japanese Patent Application Laid-open No. 8-334618, there is a method in which a retardation film is partially heated to a temperature that is no lower than the glass transition point so as to lower the retardation (degree of orientation of molecules) in this portion, as well as a method where a chemical liquid that can dissolve or inflate the retardation film is applied, and thereby, the retardation of this portion is lowered.
Furthermore, there is a method for authentication through observation via a polarizing plate, as disclosed in Japanese Patent Application Laid-open No. 2001-525080, where a latent image is formed by changing the azimuth angle of the optical axis of the retardation layer in an optical element.
In addition, in Japanese Patent Application Laid-open No. 11-42875 and Japanese Patent Application Laid-open No. 11-151877, cholesteric liquid crystal is used as means for authentication. In Japanese Patent Application Laid-open No. 11-42875, selective circular polarized light reflecting properties of cholesteric liquid crystal, and blue shift properties when the angle of view thereof is changed are used, as well as methods for utilizing such cholesteric liquid crystal alone or in combination with a hologram are proposed. In addition, in Japanese Patent Application Laid-open No. 11-151877, a technology is proposed, where a cholesteric liquid crystal layer as that disclosed in Japanese Patent Application Laid-open No. 11-42875 and a hologram image as that disclosed in U.S. Pat. No. 5,574,790 are combined.
In the case of authentication of credit cards or the like as those disclosed in U.S. Pat. Nos. 5,574,790 and 5,393,099, counterfeit of the hologram portion has become a problem. A hologram pattern is manufactured by forming a metal thin film having high reflectance, such as one of aluminum, on an uneven surface in the order of μm. In addition, a hologram pattern is visible to the eye, and in some cases, counterfeiting becomes possible with a cutting apparatus.
In the case of the above-described Japanese Patent Application Laid-open No. 2001-232978, it is disclosed that a latent image is fabricated by carrying out a thermal process on a thermotropic polymer liquid crystal layer. In the case of this system, the means for orienting polymer liquid crystal is an outer force, such as pressure, and therefore, application of high pressure or sufficient shear stress is necessary, in order to gain sufficient orientation. Accordingly, it is necessary for the orientation of liquid crystal to have birefringence within the surface, in order to gain a latent image where the retardant is modified in accordance with the heating pattern, and to do so, it is necessary to apply sufficient shear stress to the liquid crystal, so that the delay phase axis is in a particular direction within the surface in the state of liquid crystal. Therefore, pressure is applied to the base or to the liquid crystal layer while being heated, and thus, problems, such as deformation of the base or the occurrence of damage to the liquid crystal layer, arise, that is to say, a problem arises, where an engraved seal, for example, provides an uneven pattern in such a manner that the latent image becomes visible without using a polarizing plate.
Furthermore, though a latent image can be fabricated in accordance with a method as that disclosed in Japanese Patent Application Laid-open No. 8-334618, it is necessary to heat the retardation film to a temperature that is no lower than the glass transition point and maintain this temperature for a predetermined period of time or longer, in order to cease the retardation of the retardation film. As described above, the degree of molecular orientation in the retardation film is relaxed by heating the retardation film to a temperature that is no lower than the glass transition point, and a problem arises, where the latent image is made visible due to the occurrence of unevenness in the form of the surface. This is the same in the case where heating is carried out in a state of non-contact, and permanent deformation of the film occurs, due to molecular relaxation, even when there is no pressure.
Furthermore, this is the same also as the case where a chemical liquid is applied, and it is necessary to provide a high level of freedom to the polymer that forms the retardation film, because the degree of molecular orientation of the retardation film is relaxed, and as a result, deformation in the form of the surface occurs as a result of relaxation. In the case of application of a chemical liquid, though this can be controlled through permeation of the chemical liquid, the retardation cannot be made sufficiently small when deformation does not occur, because permeation is only in the surface. That is to say, a problem arises, where contrast in the latent image cannot be made sharp. Furthermore, in the case of swelling as a result of the chemical liquid, expansion of the retardation film in the direction of the width occurs at the same time as permeation into the retardation film in the direction of the thickness, and therefore, a problem arises, where sufficient resolution is not gained in the latent image that is formed of portions where the retardation has changed and portions where the retardation has not changed.
In the case of the method that is disclosed in Japanese Patent Application Laid-open No. 2001-525080, very complicated steps are required, such that an optical orientation film is formed and irradiated with ultraviolet rays that are polarized in a predetermined direction through a mask or through scanning, and after that, is irradiated with ultraviolet rays that are polarized in another direction, so that polymerizing liquid crystal or a liquid crystal polymer thin film is formed, and then, this is oriented and fixed. At this time, an optical orientation film for determining the direction in which liquid crystal is oriented is expensive, and furthermore, polymerizing liquid crystal and liquid crystal polymers are relatively expensive. Furthermore, it is necessary to prepare two light sources of polarized ultraviolet rays which are uniform and intense and are polarized in different directions, and efficiency is low and the apparatuses expensive. Liquid crystal layers are generally fabricated through an application process, and it is difficult to control the thickness of the thin film when a certain level of retardation is gained, due to the large birefringence of the liquid crystal.
In addition, in accordance with a method for applying cholesteric liquid crystal as that of Japanese Patent Application Laid-open No. 11-42875, the level of anti-counterfeit is increased by combining liquid crystal and setting of the selective wavelength reflecting band in the circular polarized light with another anti-counterfeit function, such as a hologram image as discussed in Japanese Patent Application Laid-open No. 11-151877. However, a problem arises with the reflecting properties of the cholesteric liquid crystal, where selection of the reflecting properties of the material and the mixture of the materials can be relatively easily reproduced, that is to say, when the mixing ratio of nematic liquid crystal and a chiral agent is identified, this can be easily applied. In addition, even with a combination of a cholesteric layer and a hologram layer, these are simply combined, and therefore, combination becomes easy if counterfeiting of each is easy.
The present invention is provided in view of the above-described situation, and an object thereof is to provide a sheet with highly effective anti-counterfeit functions, where counterfeiting is difficult in the case where authentication information is recorded using the properties of cholesteric liquid crystal.